Recent years, in a vehicle including an internal combustion engine, a catalyst for exhaust gas purification is disposed in an exhaust pipe, and an exhaust gas sensor (air-fuel ratio sensor or oxygen sensor) for detecting an air-fuel ratio or rich/lean state of exhaust gas is disposed on an upstream side of this catalyst or both on upstream and downstream sides of the catalyst. The air-fuel ratio is feedback controlled based on the output of this exhaust gas sensor to increase an exhaust gas purification rate by the catalyst.
In such an exhaust gas purifying system, abnormality of the exhaust gas sensor needs to be diagnosed to prevent continued operation in a state where the exhaust gas sensor deteriorates and accuracy in control of the air-fuel ratio is reduced (state in which exhaust gas purification efficiency is lowered).
As an abnormality diagnostic technique for the exhaust gas sensor (e.g., air-fuel ratio sensor), there is, for example, a technique described in JP-A-2006-322426. This technique changes an air-fuel ratio forcibly, and determines normality/abnormality of the exhaust gas sensor based on a response time of the exhaust gas sensor with respect to the change of the air-fuel ratio.
The exhaust gas sensor has characteristics that the output of the exhaust gas sensor stagnates (change speed of the output is slowed down) in stoiciometry vicinity (stoiciometry and its vicinity) due to a catalyst layer which is one of components of a sensor element.
However, according to the art in the above publication of JP-A-2006-322426, a region in which the output of the exhaust gas sensor stagnates because of the catalyst layer (hereinafter referred to as a “sensor output plateau region”) is not taken into consideration at all. Consequently, an influence of the sensor output plateau region may be included in the response time of the exhaust gas sensor. If the influence of the sensor output plateau region is included in the response time of the exhaust gas sensor, responsivity of the exhaust gas sensor cannot be accurately evaluated, and accuracy in detection of deterioration of the exhaust gas sensor responsivity may be reduced.